The T cell receptor (TCR) is a multisubunit complex which functions as an antigen recognition structure transmitting signals for proliferation of mature T lymphocytes and for thymic selection of immature thymocytes. The clonotypic antigen (Ag)/major histocompatibility complex (MHC) recognition unit, termed Ti-alpha-beta (or Ti gamma-delta) is a heterodimer associated with the nonpolymorphic CD3 subunits gamma, delta,epsilon,sigma and eta. CD3sigma and CD3eta are products of alternative splicing and form disulfide-linked homo-or heterodimers, thereby creating distinct TCR isoforms. The gamma subunit of the high affinity Fc receptor for IgE (FCepsilonRIgamma) and cD3sigma/eta have significant structural homology and likely belong to the same gene family. FcepsilonRIgamma is also an essential component of transmembrane type FcgammaRIII (CD16), the low affinity Fc receptor for IgG. Recent in vitro studies have shown that FcepsilonRIgamma family function in distinct receptor systems by forming various disulfide-linked dimers. However, it is unknown whether FcepsilonRIgamma plays a significant role in the TCR complex in vivo. We have now identified a new TCR isoform containing the FcepsilonRIgamma homodimer in lieu of CD3sigma/eta; this TCR isoform is expressed on splenic CD2+CD3/Tialpha-beta+CD4-CD8- CD16+NK1.1+ mouse large granular lymphocytes (LGL) also known as lymphokine activated killer cells (LAK) which show strong natural killer activity against tumor cells. We have also shown that Ti alpha-beta+CD4- CD8- cells found in both thymus and periphery have indistinguishable if not identical properties with LGL. Ti alpha-beta+CD4-CD8- cells are known to be abnormally expanded in autoimmune lpr/lp and gld/gld mice and human systemic lupus erythematosus (SLE) patients. The overall goal of this project is to understand the role of FcepsilonRIgamma in TCR signaling and the function of FcepsilonRIgamma in the development of LGL and Tia-beta+CD4-CD8-lymphocytes. The specific aims of this proposal include: 1) structural analysis of FcepsilonRIgamma proteins with anti- FcepsilonRIgamma antibodies raised against specific peptide sequences; 2) analysis of the signal transduction pathways through FcepsilonRIgamma- containing receptors to examine whether such receptors utilize signal transduction pathways distinct from other TCR isoforms; and 3) developmental analysis of Tia-beta+CD4-CD8-LGL by biochemical analysis of TCR complexes from different thymic developmental stages and by lineage analysis utilizing intrathymic cell transfer. These studies will elucidate the physiological role of FcepsilonRIgamma i signal transduction of TCR, in thymic development and in the TCR repertoire formation of Tia-beta+CD4-CD8-LGL. These studies will also contribute to our understanding of the role of Tia-beta+CD4-CD8- population i autoimmune diseases, and induction and activation mechanisms of LGL/LAK leading to cytotoxicity against tumor cells.